Add easier plotting options (geoms?) for events

[pratikunterwegs]

This issue is to request easier ways to add events such as vaccination regimes and interventions to plots of epidemic curves.

The current best method to do this is to manually create geom_rect or geom_vline along with annotate(geom = "text") to add representations of when these events are active, to epidemic curve plots made with ggplot2.

The new functionality could be in the form of custom geoms as documented in this book.